U.S. patent application number 10/015998 was filed with the patent office on 2002-09-05 for handoff control in an asynchronous cdma system.
Invention is credited to Terasawa, Daisuke.
Application Number | 20020122396 10/015998 |
Document ID | / |
Family ID | 23052680 |
Filed Date | 2002-09-05 |
United States Patent
Application |
20020122396 |
Kind Code |
A1 |
Terasawa, Daisuke |
September 5, 2002 |
Handoff control in an asynchronous CDMA system
Abstract
In a CDMA system in which the base stations are not each time
aligned with one another, the handoff process accommodates the
handoff by allowing for frame alignment. For example, frame
alignment may be accomplished through the use of a selected set or
may be remote unit aligned without reference to external sources.
In addition, the neighbor list may included additional entries.
Inventors: |
Terasawa, Daisuke; (San
Diego, CA) |
Correspondence
Address: |
QUALCOMM Incorporated
5775 Morehouse Drive
San Diego
CA
92121-1714
US
|
Family ID: |
23052680 |
Appl. No.: |
10/015998 |
Filed: |
December 10, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10015998 |
Dec 10, 2001 |
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09275524 |
Mar 24, 1999 |
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Current U.S.
Class: |
370/331 ;
370/503 |
Current CPC
Class: |
H04W 56/00 20130101;
H04W 36/18 20130101 |
Class at
Publication: |
370/331 ;
370/503 |
International
Class: |
H04Q 007/00 |
Claims
What is claimed is:
1. A method of handoff control for a wireless remote unit having an
established communications link with a first base station,
comprising the steps of: transmitting a message to a network
controller identifying a second base station having signal strength
sufficient to establish communication; receiving a message from
said network controller via said first base station identifying
said second base station as a selected base station; monitoring an
overhead channel from said second base station in order to
determine a frame synchronization of said second base station; and
transmitting said frame synchronization to said network
controller.
2. The method of claim 1, further comprising the step of receiving
a message from said network controller via said first base station
identifying said second base station as an active base station.
3. The method of claim 1, wherein said first base station and said
second base station are asynchronous with respect to one
another.
4. The method of claim 1, further comprising the step of diversity
combining signals transmitted by said first and said second base
stations.
5. The method of claim 1, further comprising the step of receiving
a message from said network controller via said first base station
comprising a neighbor list from which said second base station is
selected.
6. The method of claim 5 wherein said neighbor list comprises a
series of entries, each entry corresponding to a base station with
a high probability of having signal strength sufficient to
establish communication, said entries comprising information
identifying a reference base station and a PN offset wherein a
timing of said reference base station is used as a reference timing
for said PN offset.
7. The method of claim 6, wherein said entries further comprise a
window size over which a search should be performed.
8. The method of claim 7, wherein said window size implicitly
carries information concerning a relative class of synchronization
between said reference base station and said base station to which
said entries correspond.
9. The method of claim 7, wherein said window size carries
information concerning whether said base station corresponding to
said entry is frame synchronized with said reference base
station.
10. A handoff control apparatus in a wireless remote unit, said
remote unit having an established communication link with a first
base station, said apparatus comprising: means for transmitting a
message to a network controller identifying a second base station
having signal strength sufficient to establish communication; means
for receiving a message from said network controller via said first
base station identifying said second base station as a selected
base station; means for monitoring an overhead channel from said
second base station in order to determine a frame synchronization
of said second base station; and means for transmitting said frame
synchronization to said network controller.
11. A method of handoff control for a wireless remote unit having
an established communications link with a first base station,
comprising the steps of: receiving a message from said remote unit
identifying a second base station having signal strength sufficient
to establish communication; transmitting a message to said remote
unit via said first base station identifying said second base
station as a selected base station; receiving a message identifying
a frame synchronization of said second base station; and
establishing communication with said remote unit via said second
base station such that transmissions from said first base station
and transmissions from said second base station arrive at said
remote unit approximately synchronized.
12. The method of claim 11, further comprising the step of
transmitting a message to said remote unit via said first base
station identifying said second base station as an active base
station.
13. The method of claim 11, wherein said first base station and
said second base station are asynchronous with respect to one
another.
14. The method of claim 11, further comprising the step of
diversity combining signals received from said remote unit via said
first and said second base stations.
15. The method of claim 11, wherein said step of transmitting a
message identifying said second base station as a selected base
station is executed only if resources are available at said second
base station to support communication with said remote unit.
16. A wireless remote unit having an established communications
link with a first base station, comprising: means for receiving a
message from said remote unit identifying a second base station
having signal strength sufficient to establish communication; means
for transmitting a message to said remote unit via said first base
station identifying said second base station as a selected base
station; means for receiving a message identifying a frame
synchronization of said second base station; and means for
establishing communication with said remote unit via said second
base station such that transmissions from said first base station
and transmissions from said second base station arrive at said
remote unit approximately synchronized.
17. In a communication system in which a remote unit communicates
with other users via at least one base station, and in which each
of at least two or more base stations among a plurality of base
stations within said system transmits a unique pilot signal, a
remote unit transceiver comprising: a pilot signal measurement
circuit which measures strength of pilot signals received from a
set of neighboring base stations; a controller which generates a
first signal strength message when a measured pilot signal of a
target base station from among said set of neighboring base
stations exceeds a first predetermined level; a remote unit
transmitter which transmits said first signal strength message to
at least one base station with which said remote unit is currently
communicating, said first signal strength message identifying said
target base station; and a demodulator which receives a first
direction signal from said at least one base station and, in
response to said first direction signal, monitors a forward link
transmission from said target base station to determine a frame
synchronization of said target base station.
18. The remote unit transceiver of claim 17 wherein said controller
generates a relative frame synchronization message to convey said
frame synchronization and said remote unit transmitter transmits
said relative frame synchronization to said at least one base
station.
19. In a spread spectrum communication system having a plurality of
base stations and in which a remote unit communicates with another
system user via at least one base station, a method for directing
communications between said remote unit and said base stations
comprising the steps of: providing to said remote unit an active
list identifying one or more base stations through which active
communication is established; receiving from said remote unit a
candidate list identifying at least one target base station;
determining an availability of system resources at said target base
station; providing to said remote unit a selected list identifying
said target base station; receiving from said remote unit an
alignment message identifying synchronization information
concerning said target base station; directing said target base
station to establish communication with said remote unit in
accordance with said synchronization information; and providing to
said remote unit a second active list identifying said target base
station.
20. The method of claim 19, wherein said selected list comprises
just one entry.
21. The method of directing communications of claim 19, further
comprising the steps of providing to said remote unit a neighbor
list comprising a series of entries, said entries comprising
information identifying a reference base station and a PN offset
wherein a timing of said reference base station is used as a
reference timing for said PN offset.
22. The method of directing communications of claim 21, wherein
said entries further comprise a window size over which a search
should be performed.
23. The method of directing communications of claim 22, wherein
said window size implicitly carries information concerning a
relative class of synchronization between said reference base
station and said base station to which said entries correspond.
24. The method of directing communications of claim 22, wherein
said window size carries information concerning whether said base
station corresponding to said entry is frame synchronized with said
reference base station.
25. A network controller in spread spectrum communication system in
which a remote unit communicates with another system user via at
least one base station and wherein each base station transmits an
identifying pilot signal, said network controller comprising: means
for providing to said remote unit an active list identifying one or
more base stations through which active communication is
established; means for receiving from said remote unit a candidate
list identifying at least one target base station; means for
determining an availability of system resources at said target base
station; means for providing to said remote unit a selected list
identifying said target base station; means for receiving from said
remote unit an alignment message identifying synchronization
information concerning said target base station; means for
directing said target base station to establish communication with
said remote unit in accordance with said synchronization
information; and means for providing to said remote unit a second
active list identifying said target base station.
26. A method of time alignment in a wireless communications system
in which a remote unit is capable of communication with one or more
base stations simultaneously, said method comprising the steps of:
receiving a first forward link transmission from a first base
station having a first frame alignment; receiving a second forward
link transmission from a second base station having a second frame
alignment wherein said second frame alignment comprises information
concerning frame boundaries and excludes information concerning an
absolute frame number; selecting a first arbitrary frame alignment;
combining said first forward link transmission and said second
forward link transmission according to said first arbitrary frame
alignment creating a combined signal; determining whether a
performance indication of said combined signal is within expected
limits; and combining said first forward link transmission and said
second forward link transmission using a second arbitrary frame
alignment if said performance indication is not within the expected
limits.
27. A method of time alignment in a wireless communications system
in which a remote unit is capable of communication with one or more
base stations simultaneously, said method comprising the steps of:
receiving a first forward link transmission from a first base
station having a first frame alignment; receiving a second forward
link transmission from a second base station having a second frame
alignment wherein said second frame alignment comprises information
concerning frame boundaries and excludes information concerning an
absolute frame number; combining said first forward link
transmission and said second forward link transmission according to
a first frame alignment hypothesis to determine a first performance
indication; combining said first forward link transmission and said
second forward link transmission according to a second frame
alignment hypothesis to determine a second performance indication;
and comparing said first and second performance indications in
order to determine a most likely absolute frame alignment.
28. An apparatus for time alignment in a wireless communications
remote unit capable of communication with one or more base stations
simultaneously, said apparatus comprising: means for receiving a
first forward link transmission from a first base station having a
first frame alignment; means for receiving a second forward link
transmission from a second base station having a second frame
alignment wherein said second frame alignment comprises information
concerning frame boundaries and excludes information concerning an
absolute frame number; means for selecting a first arbitrary frame
alignment; means for combining said first forward link transmission
and said second forward link transmission according to said first
arbitrary frame alignment; means for determining whether a
performance indication is within expected limits; and means for
combining said first forward link transmission and said second
forward link transmission using a second arbitrary frame alignment
if said performance indication is not within the expected
limits.
29. An apparatus for time alignment in a wireless communications
system in which a remote unit is capable of communication with one
or more base stations simultaneously, said apparatus comprising:
means for receiving a first forward link transmission from a first
base station having a first frame alignment; means for receiving a
second forward link transmission from a second base station having
a second frame alignment wherein said second frame alignment
comprises information concerning frame boundaries and excludes
information concerning an absolute frame number; means for
combining said first forward link transmission and said second
forward link transmission according to a first frame alignment
hypothesis to determine a first performance indication; means for
combining said first forward link transmission and said second
forward link transmission according to a second frame alignment
hypothesis to determine a second performance indication; and means
for comparing said first and second performance indications in
order to determine a most likely absolute frame alignment.
30. In a spread spectrum communication system in which a remote
unit communicates with another system user via at least one base
station, a method for directing communications between said remote
unit and said base stations comprising the steps of: providing to
said remote unit a neighbor list identifying one or more base
stations; providing to said remote unit an active list identifying
one or more base stations through which active communication is
established; receiving from said remote unit a candidate list
identifying at least one target base station; determining an
availability of system resources at said at least one target base
station; and providing to said remote unit an active list
identifying said at least one target base station; wherein said
neighbor list comprises a series of entries, said entries
comprising information identifying a reference base station and a
PN offset wherein a timing of said reference base station is used
as a reference timing for said PN offset.
31. The method of claim 30, wherein said entries further comprise a
window size over which a search should be performed.
32. The method of claim 31, wherein said window size implicitly
carries information concerning a relative class of synchronization
between said reference base station and said base station to which
said entries correspond.
33. The method of claim 31, wherein said window size carries
information concerning whether said base station corresponding to
said entry is frame synchronized with said reference base
station.
34. In a spread spectrum communication system in which a remote
unit communicates with another system user via at least one base
station, an apparatus for directing communications between said
remote unit and said base stations comprising: means for providing
to said remote unit a neighbor list identifying one or more base
stations; means for providing to said remote unit an active list
identifying one or more base stations through which active
communication is established; means for receiving from said remote
unit a candidate list identifying at least one target base station;
means for determining an availability of system resources at said
at least one target base station; and means for providing to said
remote unit an active list identifying said at least one target
base station; wherein said neighbor list comprises a series of
entries, said entries comprising information identifying a
reference base station and a PN offset wherein a timing of said
reference base station is used as a reference timing for said PN
offset.
35. In a spread spectrum communication system in which a remote
unit communicates with another system user via at least one base
station, a method for directing communications between said remote
unit and said base stations comprising the steps of: receiving at
said remote unit an active list identifying one or more base
stations through which active communication is established;
receiving at said remote unit a neighbor list identifying one or
more base stations; measuring at said remote unit a signal strength
of a pilot signal transmitted by each base station having an entry
on said neighbor list; transmitting a first message from said
remote unit, said first message identifying a candidate list
comprising an entry corresponding to at least one target base
station; and receiving at said remote a new active list of base
station comprising an entry corresponding to said at least one
target base station; wherein said neighbor list comprises a series
of entries, said entries comprising information identifying a
reference base station and a PN offset wherein a timing of said
reference base station is used as a reference timing for said PN
offset.
36. The method of claim 35, wherein said entries further comprise a
window size over which a search should be performed.
37. The method of claim 36, wherein said window size implicitly
carries information concerning a relative class of synchronization
between said reference base station and said base station to which
said entries correspond.
38. The method of claim 36, wherein said window size carries
information concerning whether said base station corresponding to
said entry is frame synchronized with said reference base
station.
39. In a spread spectrum communication system in which a remote
unit communicates with another system user via at least one base
station, an apparatus for directing communications between said
remote unit and said base stations comprising: means for receiving
at said remote unit an active list identifying one or more base
stations through which active communication is established; means
for receiving at said remote unit a neighbor list identifying one
or more base stations with a high probability of having signal
strength sufficient to establish communication; means for measuring
at said remote unit a signal strength of a pilot signal transmitted
by each base station having an entry on said neighbor list; means
for transmitting a first message from said remote unit, said first
message identifying a candidate list comprising an entry
corresponding to at least one target base station; and means for
receiving at said remote a new active list of base station
comprising an entry corresponding to said at least one target base
station; wherein said neighbor list comprises a series of entries,
said entries comprising information identifying a reference base
station and a PN offset wherein a timing of said reference base
station is used as a reference timing for said PN offset.
Description
BACKGROUND OF THE INVENTION
[0001] I. Field of the Invention
[0002] The invention relates generally to wireless communications.
More particularly, the invention relates to handoff control in a
wireless communication system.
[0003] II. Description of the Related Art
[0004] FIG. 1 is an exemplifying embodiment of a terrestrial
wireless communication system 10. FIG. 1 shows the three remote
units 12A, 12B and 12C and two base stations 14. In reality,
typical wireless communication systems may have many more remote
units and base stations. In FIG. 1, the remote unit 12A is shown as
a mobile telephone unit installed in a car. FIG. 1 also shows a
portable computer remote unit 12B and the fixed location remote
unit 12C such as might be found in a wireless local loop or meter
reading system. In the most general embodiment, remote units may be
any type of communication unit. For example, the remote units can
be hand-held personal communication system units, portable data
units such as a personal data assistant, or fixed location data
units such as meter reading equipment. FIG. 1 shows a forward link
signal 18 from the base stations 14 to the remote units 12 and a
reverse link signal 20 from the remote units 12 to the base
stations 14.
[0005] An industry standard for a wireless system using code
division multiple access (CDMA) is set forth in the TIA/EIA Interim
Standard entitled "Mobile Station--Base Station Compatibility
Standard for Dual-Mode Wideband Spread Spectrum Cellular System",
TIA/EIA/IS-95, and its progeny (collectively referred to here in as
IS-95), the contents of which are also incorporated herein by
reference. More information concerning a code division multiple
access communication system is disclosed in U.S. Pat. No.
4,901,307, entitled "SPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION
SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS," assigned to the
assignee of the present invention and incorporated in its entirety
herein by this reference.
[0006] In an IS-95 system, each base station synchronizes its
operation with other base stations in the system. For example, in
one embodiment, the IS-95 base stations synchronize operation to a
universal time reference such as Global Positioning Satellites
(GPS) signaling. Based upon the synchronizing time reference, each
base station in a given geographical area is assigned a sequence
offset of a common pseudo noise (PN) pilot sequence. For example,
according to IS-95, a PN sequence having 2.sup.15 chips and
repeating every 26.66 milliseconds (ms) is transmitted by each base
station in the system at one of 512 PN sequence offsets as a pilot
signal. According to IS-95 operation, the base stations continually
transmit the pilot signal which can be used by the remote units to
identify the base stations as well as for other functions.
[0007] Various methods exist for transferring communication with
the remote unit from one base station to another through a process
known as handoff. Handoff may be necessary if a remote unit
operating in the coverage area of an original base station moves
into the coverage area of a target base station. One method of
handoff used in CDMA systems is termed a "soft" handoff. Through
the use of soft handoff, communication with the target base station
is established before termination of communication with the
original base station. When the remote unit is communicating with
two base stations, both the remote unit and base stations create a
single signal from the multiple received signals. Through the use
of soft handoff, communication between the remote unit and the end
user is uninterrupted by the eventual handoff from the original
base station to the target base station. U.S. Pat. No. 5,267,261
entitled "MOBILE STATION ASSISTED SOFT HANDOFF IN A CDMA CELLULAR
COMMUNICATIONS SYSTEM," which is assigned to the assignee of the
present invention and which is incorporated herein, discloses a
method and system for providing communication with the remote unit
through more than one base station during the handoff process.
Further information concerning handoff is disclosed in U.S. Pat.
No. 5,101,501, entitled "METHOD AND SYSTEM FOR PROVIDING A SOFT
HANDOFF IN COMMUNICATIONS IN A CDMA CELLULAR TELEPHONE SYSTEM,"
U.S. Pat. No. 5,640,414, entitled "MOBILE STATION ASSISTED SOFT
HANDOFF IN A CDMA CELLULAR COMMUNICATIONS SYSTEM," and U.S. Pat.
No. 5,625,876 entitled "METHOD AND APPARATUS FOR PERFORMING HANDOFF
BETWEEN SECTORS OF A COMMON BASE STATION," each of which is
assigned to the assignee of the present invention and incorporated
in its entirety herein by this reference. The subject matter of
U.S. Pat. No. 5,625,876 concerns so-called "softer handoff." For
the purposes of this document, the term "soft handoff" is intended
to include both "soft handoff" and "softer handoff."
[0008] As described in the above mentioned patent, remote unit
assisted soft handoff operates based on the pilot signal strength
of several sets of base stations as measured by the remote unit:
the active set, the neighbor set, the candidate set and remaining
set. The active set is the set of base stations through which
active communication is established. The neighbor set is a set of
base stations surrounding the active base stations and comprising
base stations that have a high probability of having a pilot signal
strength of sufficient level to establish communication. The
candidate set is a set of base stations having a pilot signal
strength of sufficient level to establish communication but through
which active communication is not yet established. The remaining
set is a set of base stations which are not in any of the other
three sets.
[0009] The remote unit uses these sets to control the handoff
process. In this example, we shall assume that when communications
are initially established, a remote unit communicates through a
first base station, and the active set contains only the first base
station although in many cases, the active set contains more than
one base station before the handoff process is begun with respect
to yet another base station. The remote unit monitors the pilot
signal strength of the base stations in the active set, the
candidate set, the neighbor set and the remaining set. When a pilot
signal strength of a base station in the neighbor or remaining set
exceeds a predetermined threshold level, the base station is added
to the candidate set and removed from the neighbor or remaining set
at the remote unit. The remote unit communicates a pilot strength
measurement overhead message through the first base station
identifying the new base station. A system controller receives the
pilot strength measurement overhead message from the first base
station and decides whether to establish communication between the
new base station and the remote unit. Should the system controller
decide to do so, the system controller sends a message to the new
base station with identifying information about the remote unit and
a command to establish communications with the remote unit.
[0010] A handoff message is also transmitted to the remote unit
through the first base station. The handoff message is an overhead
message which identifies a new active set that includes the first
and the new base stations. The handoff message also identifies
which channel has been allocated for use by the remote unit with
the new base station. The remote unit searches for the new base
station's transmitted signal, and communication is established with
the new base station without termination of communication through
the first base station. This process can continue with additional
base stations such that two or more base stations are in the active
set.
[0011] When the remote unit is communicating through multiple base
stations, it continues to monitor the signal strength of the base
stations of the active set, the candidate set, the neighbor set and
the remaining set. Should the signal strength corresponding to a
base station of the active set drop below a predetermined threshold
for a predetermined period of time, the remote unit generates and
transmits an overhead message to report the event. The system
controller receives this message through at least one of the base
stations with which the remote unit is communicating. In response
to receiving this message, the system controller can decide to
terminate communications through the base station having a weak
signal strength.
[0012] Upon forming a decision to terminate communications through
a base station, the system controller generates a handoff message
identifying a new active set of base stations. The new active set
does not contain the base station through which communication is to
be terminated. The base stations through which communication is
established send the handoff message to the remote unit with the
new active set. The remote unit receives the overhead message and
removes the base station from the active set and, typically, places
it in the neighbor set. The remote unit communications are, thus,
routed only through base stations identified in the new active
set.
[0013] Because the remote unit is communicating with the end user
through at least one base station at all times throughout the soft
handoff process, no interruption in communications occurs between
the remote unit and the end user. A soft handoff provides
significant benefits in its inherent "make before break"
communication over conventional "break before make" (hard handoff)
techniques employed in cellular communication systems employing
other multiple access techniques such as time division multiple
access (TDMA) or frequency modulation (FM).
[0014] As noted above, each base station is associated with a set
of neighboring base stations surrounding the base station. Due to
the close physical proximity of the coverage areas of the
neighboring base stations to the coverage area of the active base
station, the remote units which are communicating with the active
base station are more likely to handoff to one of the neighboring
base stations than to other base stations in the system. The base
station identifies the neighboring base stations to the remote
units with which it has established communication using a neighbor
list identification message. The neighbor list identification
message identifies a neighboring base station according to the PN
sequence offset at which it transmits the pilot signal.
[0015] Due to path delays and multipath, the relative time offset
between two pilot signals arriving at a remote unit from
neighboring base stations is typically not identically equal to the
nominal PN sequence offset. In addition, the delay and the
multipath environment are constantly changing due to the relative
movement of objects within the base station coverage areas.
Therefore, a searching element within the remote unit is used to
search for the pilot signals of the neighboring base stations over
a range of PN sequence offsets relative to known timing
conditions.
[0016] Each search which the searching element performs can be
characterized as having a nominal PN sequence offset and a
corresponding search window. The search window specifies a set of
PN sequence offsets around the nominal PN sequence offset.
Generally, the search window comprises a range of offsets in which
the remote unit is likely to detect a pilot signal. For each offset
processed, the searching element finds the correlation energy at
that offset by despreading the antenna samples using the same PN
sequence used to generate the pilot signal. The searching element
accumulates the energy for a period of time and reports the
accumulated energy to a remote unit controller. If the accumulated
energy exceeds a certain threshold, a pilot signal is detected.
[0017] The remote unit uses the neighbor list to limit the space
over which it searches for handoff candidates. For example, because
the searching process is so resource intensive, it is advantageous
to avoid performing a search over the entire set of possible PN
sequence offsets. By using the neighbor list, the remote unit can
concentrate its resources on those PN sequence offsets which are
most likely to correspond to useful signal paths.
[0018] The IS-95 operation is practical so long as each base
station's timing remains synchronous with respect to the others.
However, in some systems, advantages are achieved by decoupling
operation of the system from a synchronizing reference. For
example, in a system which is deployed underground, such as in a
subway system, it can be difficult to derive a universal time
synchronization signal using GPS. In addition, in certain political
climates, it is perceived as desirable to decouple system operation
from signaling under the control of another political entity.
[0019] In a system where one or more of the base stations operate
asynchronously with respect to other base stations in the system,
the base stations cannot be distinguished from one another based
upon a relative time offset (typically measured as a relative PN
sequence offset) because a relative time offset between the base
stations cannot be established without the use of a universal time
reference. Therefore, the handoff control systems just described
must be modified to accommodate asynchronous operation.
[0020] Thus, there is a need in the art to develop a handoff
control mechanism for use in an asynchronous CDMA system.
SUMMARY OF THE INVENTION
[0021] In one embodiment, in a system where a wireless remote unit
is capable of communication with multiple base stations
simultaneously, handoff to a subsequent base station is controlled
through use of a selected set of base stations. For example, if
frame synchronization between a second base station and the active
base station is unknown, when a second base station has signal
strength sufficient to establish communication, the remote unit
transmits a message to a network controller identifying the second
base station. The network controller determines an availability of
resources at the second base station. If resources are available,
the base station sends a message via the first base station
identifying the second base station as a selected base station. In
response, the remote unit monitors an overhead channel from the
second base station in order to determine a frame synchronization
of the second base station. The remote unit transmits the frame
synchronization to the network controller. The network controller
commands the second base station to begin transmission of signals
to the remote unit such that the transmission from the second base
station arrives at the remote unit approximately synchronized with
the transmission from the first base station.
[0022] In another embodiment, time alignment in a wireless
communications system is achieved by the remote unit without using
the round trip messages of the selected set embodiment. For
example, the remote unit receives a forward link transmission from
a first base station having a first frame alignment. The remote
unit receives a second forward link transmission from a second base
station having a second frame alignment. The second frame alignment
comprises information concerning frame boundaries but not
information concerning an absolute frame number. The remote unit
combines the first forward link transmission and the second forward
link transmission according to a first frame alignment. The remote
unit determines whether a performance indication is within expected
limits. If the performance indication is not within the expected
limits, the remote unit combines the first forward link
transmission and the second forward link transmission using a
second frame alignment.
[0023] In yet another embodiment, a composite neighbor list is
created by the remote unit or the network controller for use during
the handoff process. For example, in the case of the remote unit
determined composite neighbor list, the remote unit receives one or
more neighbor lists corresponding to one or more active base
stations. The remote unit removes entries from the one or more
neighbor lists corresponding to base stations through which active
communication is established. The remote unit aligns a time offset
reference of at least one entry in the neighbor lists so that the
entries in the neighbor lists are referenced to a common timing
reference. For each base station having more than one entry on the
neighbor list, the remote unit determines a single composite entry
specifying a composite search window equal to the intersection of a
search window specified in each entry corresponding to the base
station.
[0024] In yet a further embodiment, the neighbor list used to
facilitate handoff comprises a series of entries. The entries
comprise information identifying a reference base station and a PN
offset. A timing of the reference base station is used as a
reference timing for the PN offset. The entries may further
comprise a window size over which a search should be performed. The
window size may implicitly carry information concerning a relative
class of synchronization between the reference base station and the
base station to which the entries correspond. For example, the
window size may carry information concerning whether the base
station corresponding to the entry is frame synchronized with the
reference base station.
BRIEF DESCRIPTION OF THE DRAWINGS
[0025] The features, objects and advantages of the present
invention will become more apparent from the detailed description
set forth below when taken in conjunction with the drawings in
which like reference characters identify correspondingly
throughout, and wherein:
[0026] FIG. 1 is a block diagram showing a typical modern wireless
communication system.
[0027] FIG. 2 is a block diagram showing a set of base stations in
an asynchronous system.
[0028] FIG. 3 is a flow chart illustrating neighbor list
combination.
[0029] FIG. 4 is a flow chart illustrating the use of a selected
set in a system comprising asynchronous base stations.
[0030] FIG. 5 is a block diagram of a remote unit.
[0031] FIG. 6 is a flow chart illustrating remote unit assisted
frame alignment.
DETAILED DESCRIPTION OF THE INVENTION
[0032] In an asynchronous code division multiple access (CDMA), no
universal time reference is used to synchronize the operation of
the base stations. In some cases, groups of base stations are
synchronized to a single reference. For example, a set of base
stations deployed within a single building can be synchronized to a
common timing source. In addition, sectors within a base station
are typically synchronized to a common source. However, the
relative timing between other base stations in the system is
unknown or less than purely synchronous.
[0033] FIG. 2 is a block diagram showing a set of base stations in
an asynchronous system. FIG. 2 shows a multi-sectored base station
having sectors labeled 40A, 40B and 40C. In this document, the term
base station may be used to refer to a single sectored base
station, a sector of a multisectored base station, or a
multi-sectored base station. Using this terminology, in this case
we shall assume that the base stations 40A-40C are sectors in a
common multisectored base station and operate from a common time
reference so as to be purely synchronous with respect to one
another.
[0034] The system shown in FIG. 2 also comprises base stations 42,
44 and 46. The base stations 42, 44 and 46 are synchronized to some
lesser extent than the base stations 40A-40C. The system also
comprises base stations 48 and 50. The base stations 48 and 50
operate asynchronously with respect to one another as well as with
respect to the base stations 40A-40C and the base stations 42-46.
The base stations are controlled by a network controller 60. The
base stations 42-50 are each assigned a geographically unique PN
sequence with which to generate a pilot signal as well as other
signals. One cycle of the PN sequence is referred to as a frame. A
logical connection (not shown) couples each of the base stations
40-50 to the network controller 60. During soft handoff, the
network controller 60 merges the signals received from the remote
unit via the active base stations. Additional information
concerning base station and network controller operation is
disclosed in U.S. Pat. No. 5,654,979 entitled "CELL SITE
DEMODULATION ARCHITECTURE FOR A SPREAD SPECTRUM MULTIPLE ACCESS
COMMUNICATION SYSTEMS," and in U.S. Pat. No. 5,490,165 entitled
"DEMODULATION ELEMENT ASSIGNMENT IN A SYSTEM CAPABLE OF RECEIVING
MULTIPLE SIGNALS," each of which is assigned to the assignee of the
present invention and incorporated in its entirety herein by this
reference.
[0035] Another one of the tasks executed by the network controller
60 is the generation of a neighbor list for the remote units (such
as a remote unit 52) operating in the system. One purpose of the
neighbor list is to limit the search space over which a remote unit
attempts to find pilot signals transmitted by the base stations.
The neighbor list information can be transmitted to a remote unit
from the network controller 60 via one or more base stations over a
remote-unit-specific or broadcast channel.
[0036] As noted above, in an IS-95 system, each base station system
is assigned a unique PN sequence offset of a common PN sequence.
Thus, in an IS-95 system, the neighbor list identifies the
neighboring base stations according to relative PN offset. However,
in an asynchronous system, the relative time offsets of some base
stations are unknown and, thus, the base stations cannot be
identified by a relative PN offset.
[0037] For this reason, in the asynchronous system shown in FIG. 2,
multiple PN sequences are used to identify some of the base
stations in the system. Relative PN offsets can be used to
distinguish between the base stations which are purely synchronized
with one another. For example, in FIG. 2, the base stations 40A-40C
are each assigned a unique one of three different PN offsets of a
common PN sequence. The base stations 42-46 and the base stations
48 and 50 are each assigned a unique PN sequence. Given the use of
multiple PN sequences, the use of a neighbor list to limit the
search space becomes even more critical because it can be
impractical to search over a plurality of PN sequences at all
possible offsets in a timely fashion. In addition, because the
entire period of the PN sequence must be searched, it is
advantageous to limit the length of the PN sequence in order to
limit the size of the search window. However, if the duration of
each PN chip remains fixed and the length of the PN sequence is
limited, the PN sequence repeats more frequently. For example, it
can be advantageous to limit the period of PN sequences used in the
system to approximately 10 ms or shorter although longer periods
can be used.
[0038] As noted above with reference to FIG. 2, in a system where
synchronous base stations and asynchronous base stations co-exist,
several classes of synchronization can be used. For example, if GPS
timing or some other reliable timing source is available,
near-perfect synchronization can be achieved. Although it is less
accurate in general, timing can also be transferred over the
backhaul which connects the base station and the network
controller. Other timing references include but are not limited to
television and radio signaling and round trip delay reports from
remote units. In some cases, a base station that was at one time
coupled to a timing reference (such as GPS timing) loses connection
to the timing reference and begins to run based upon its own
internal timing source. In such a case, the degree of
synchronization with the other base stations deteriorates gradually
over time and eventually becomes asynchronous. For these reasons
and others, base stations within a system can be classified as
having one of several classes of synchronization.
[0039] In FIG. 2, the base stations 40A-40C can be classified as
purely synchronous with respect to one another. Purely synchronous
refers to base station timing which is known with a reasonably
small uncertainty such as, for example, less than the duration of
about 3 PN chips. If the relative timing of the base stations 42,
44 and 46 can be only determined within a larger window, they can
be classified as frame synchronous. So, for example, if the
relative timing of the base stations 42-46 can be determined within
plus or minus one half of the duration of the PN sequence (i.e. one
half the duration of the length of one frame) with respect to one
another, they are considered frame synchronous.
[0040] The base stations 48 and 50 can be classified as
asynchronous meaning that the relative timing of these base
stations is unknown with respect to any other. In addition, we
shall assume that the timing relationship of the base stations
42-46 with respect to the base station 40A-40C is also unknown.
[0041] Table I summaries the relative synchronization state between
each base station shown in FIG. 2.
1TABLE I Key S Synchronous within 3 PN chips F Synchronous within
plus or minus one half frame A Asynchronous or unknown
synchronization 40A 40B 40C 42 44 46 48 50 40A S S A A A A A 40B S
S A A A A A 40C S S A A A A A 42 A A A F F A A 44 A A A F F A A 46
A A A F F A A 48 A A A A A A A 50 A A A A A A A
[0042] Note that although Table I and the text above refer to three
different synchronization classes, many intermediate
synchronization classes between purely synchronous and asynchronous
can be specified. For example if a base station X is synchronized
to a base station Y within time duration T1 and the base station X
is synchronized to a base station Z within a time duration T2, the
base station Y and the base station Z are synchronized within a
time duration T1=T2, thus, defining several classes of
synchronization.
[0043] If a remote unit has established an active communication
link with the base station 40B, it is likely that the base stations
40A and 40C as well as the base station 48 are members of the
neighbor list. Note that even though the base stations 40A-40C are
purely synchronous with respect to one another, the relative
synchronization of the base station 40B with the base station 48 is
unknown. Thus, the base station 40B is asynchronous with respect to
base station 48.
[0044] One way in which the search space can be limited is with
reference to the relative synchronization class of the neighboring
base stations. For example, if a remote unit has established
communications with the base station 40A, it is very likely that
the base stations 40B and 40C are members of the neighbor set.
Knowing that the base stations 40A-40C are purely synchronous with
respect to one another, the search space can be limited to a
relatively small search window surrounding the relative nominal PN
sequence offset in a similar manner as the prior art.
[0045] If a remote unit establishes an active communications link
with the base station 44, the base stations 42 and 46 are likely to
be in the neighbor set. Because each of these base stations uses a
different PN sequence, in one example, the search window extends
over each possible offset of the entire PN sequence. However,
because these base stations are frame synchronous, the process of
handoff as described in detail below is simplified to some
extent.
[0046] If a remote unit has established an active communications
link with the base station 48, the base stations 40A-40C are likely
to be on the neighbor list. Because the base station 48 operates
asynchronously with respect to the base stations 40A-40C, the
search window must extend over each possible offset of the entire
common PN sequence used by the base stations 40A-40C. In addition,
if a remote unit has established an active communications link with
the base station 48, it is likely that the base stations 42-46 are
listed in the neighbor set. Once again, because the base station 48
operates asynchronously with respect to the base stations 42-46,
all three PN sequences used in these base stations must be searched
over each possible offset of the entire PN sequences. As discussed
in further detail below, because the base station 48 is not even
frame synchronous with the other base stations, additional handoff
procedures are incorporated into the handoff process.
[0047] In order to communicate search space information to the
remote unit, the neighbor list messages generated by the network
controller 60 in an asynchronous system include additional
information over and above that which was included in the IS-95
neighbor list message. One way in which such information can be
incorporated into the neighbor list message is to implicitly convey
the relative synchronization information in a search window
parameter associated with each PN sequence designated in the
neighbor list. For example, rather than explicitly specifying the
relative class of synchronization, the search window designation
can designate a limited PN search window, the entire PN period or
infinity. If the active base station and a member of the neighbor
set are purely synchronous, a search window can be specified which
designates a portion of the entire PN period in much the same
manner as an IS-95 system. If the active base station and a member
of the neighbor set are frame synchronous with respect to one
another, the neighbor list message can designate that the search
window comprises the entire PN period. If the base station with
which active communication is established is asynchronous with
respect to a base station which is a member of the neighbor set,
the search window can be set to infinity. In this way, the remote
unit can distinguish between a base station with which it is purely
synchronous, a base station with which it has frame synchronization
and a base station with which it is asynchronous. In systems with
intermediate synchronization classifications between purely
synchronous and frame synchronous, a variety of search window sizes
can be used depending on the relative class of synchronization. In
other embodiments, the relative synchronization class of the
neighboring base stations can be explicitly designated in the
neighbor list message.
[0048] As noted above, the neighbor list is used to facilitate soft
handoff between the base stations. After a remote unit has
established soft handoff with two or more base stations, the
neighbor list associated with each of the active base stations is
combined into one list by the network controller or by the remote
unit as explained more fully below. In an IS-95 synchronous system,
a combined neighbor list can be generated by simply taking the
union of the neighbor lists associated with each active base
station. Thus, an overall composite neighbor list is created from
the individual neighbor list associated with each active base
stations. According to IS-95, the composite neighbor list simply
specifies the relative PN sequence offset of each member of each
neighbor list associated with each active base station.
[0049] In an asynchronous system, there is no single timing
reference and the generation of an efficient composite neighbor
list cannot be accomplished by taking a simple union of the
entries. For example, as noted above, if a remote unit has an
active communication link established with the base station 48, the
relative classification of the base stations 40A-40C is
asynchronous. However, should the remote unit enter soft handoff
between the base station 48 and 40B, a relative synchronization
between the base station 48 and the base station 40A and 40C can be
determined because the base stations 40A-40C are purely
synchronous. Thus, when the remote unit enters soft handoff with
the base station 40B, it is no longer necessary to search the
entire period of the PN sequence used by the base stations 40A-40C
because the nominal relative PN sequence offset is known. Thus, it
is advantageous to take advantage of this information in order to
further limit the search space.
[0050] One way in which the search space can be limited is to
transmit to the remote unit a distinct neighbor list including
synchronization information for each active base station. Thus, the
remote unit may receive several entries corresponding to a
particular base station. The remote unit uses the intersection of
the set of search windows specified for a particular base station
as the search space for that base station.
[0051] Another way in which the search space can be limited is to
allow the network controller to combine the neighbor lists
associated with each active base station in order to determine a
custom composite neighbor list for the remote unit in soft handoff.
For example, the network controller is aware of the active base
stations and can designate one of the active base stations as the
timing reference. The network controller can then determine the
smallest possible search window using the synchronization
information available to the remote unit. The network controller
then combines the individual neighbor lists using the relative
classes of synchronization. The remote unit uses the timing of the
designated base station as the reference for determining the search
space.
[0052] In yet another embodiment, these two approaches are
combined. The network controller creates a customized neighbor list
and transmits it to the remote unit. The remote unit further
customizes the network controller generated list based upon the
timing information it has. For example, the remote unit can further
modify the neighbor list based upon a known relative timing between
two base stations on the neighbor list. All three of these
embodiments are more clearly understood with reference to the
following example.
[0053] According to IS-95B, the neighbor list specifies the PN
sequence offset used by the neighboring base station. In an
asynchronous system, the neighbor list must also specify the PN
code used by the base station. According to IS-95B, the PN sequence
offsets are designated with a resolution of 64 PN chips. In an
asynchronous system, a finer or more coarse resolution can be
used.
[0054] According to IS-95B, in addition to specifying the PN
sequence offset used by the neighboring base station, the neighbor
list can also specify a search window size. The size of the search
window can vary based upon the expected physical size of the active
and neighboring base station coverage areas. Note that even in a
fully synchronous system, some uncertainty in the relative PN
sequence offset of synchronous base stations is expected due to the
unknown propagation delays between the base stations and the remote
unit.
[0055] In an asynchronous system, the neighbor list can be modified
to include additional information. For example, as noted above, the
range of search windows can be specified to include infinity or the
entire PN period. In addition, the neighbor list specifies a
reference to which the search window is specified. For example, if
the remote unit has a base station in the active set which is
synchronized to some extent with the neighboring base station, the
neighbor list can specify a PN sequence offset with reference to
the timing of the signal received by the active base station.
[0056] Thus, in an asynchronous system, the neighbor list entries
contain (among other entries) the information listed in Table II
below.
2TABLE II Designation Description B. S. ID # A base station
identifier that is uniquely associated with the base station for
this geographic area. Reference A reference base station (selected
from the active set), B. S. the timing of which is used as the
reference timing for the PN offset entry. PN Offset The center (or
other known orientation) of the search window, relative to the time
of the reference base station Window Size The size of the window of
PN offsets to be searched PN Code The PN code transmitted by the
identified base station
[0057] So, for example, assume that the remote unit 52 of FIG. 2
has an active communication link established with the base station
40B. The neighbor list might include, among other information, the
information given in Table III.
3TABLE III Reference B. S. ID # B. S. PN Offset Window Size PN Code
40A 40B 459 128 3 40C 40B 873 128 3 44 40B 0 Infinity 9 46 40B 0
Infinity 11 48 40B 0 Infinity 2
[0058] The information in Table III informs the remote unit 52 that
it is likely to find the pilot signal from the base station 40A by
performing a search using PN code #3 and searching the 128 PN
sequence offsets surrounding PN offset 459 or, stated another way,
searching PN offsets of 395 through 523 relative to the signal from
the base station 40B. Similarly, the remote unit 52 that it is
likely to find the pilot signal from the base station 40C by
performing a search using PN code #3 and searching the 128 PN
sequence offsets surrounding PN offset 873 or, stated another way,
searching the PN sequence offsets of 809 through 937 relative to
the signal from the base station 40B. In addition, the remote unit
52 is likely to find the pilot signal from the base stations 44, 46
and 48 by performing a searching using PN codes #9, #11 and #2,
respectively, and searching over all PN sequence offsets.
[0059] Note that in practice, instead of specifying a window size
and PN sequence offset with a resolution of one PN chip, a more
coarse resolution can be used. More coarse designations can be used
to reduce the number of bits needed to specify the neighbor list
entry. For example, rather than allowing the window size to be
specified to be any number between 1 and 32,768 or infinity which
requires specification of 32,769 possible values and requires 16
bits of information, the set of possible window sizes can be
limited. For example, the window size could be one of 32, 54, 126,
256, 512, 1024, 32768 PN chips or infinity which can be specified
in only 3 bits.
[0060] In order to illustrate how neighbor lists from multiple
active base stations can be combined, assume that neighbor list for
the base station 44 is specified in Table IV below.
4TABLE IV Reference B. S. ID # B. S. PM Offset Window Size PN Code
48 44 0 Infinity 2 46 44 4096 2048 11 42 44 5120 256 9 40B 44 0
Infinity 3
[0061] In this case, the base stations 48 and 40B are asynchronous
with respect to the base station 44. The base station 44 has a
level of synchronization with the base station 46 such that the
receive timing uncertainty due to propagation delay uncertainty
plus uncertainty due to imperfect synchronization is 2048 chips.
The base station 42 has a level of synchronization with the base
station 44 such that the receive timing uncertainty due to
propagation delay uncertainty plus uncertainty due to imperfect
synchronization is 256 chips. In general, a small window size
suggests a higher degree of synchronization, but the
synchronization information is not expressly conveyed in the
neighbor list message explicitly.
[0062] Now consider the case where the remote unit is in soft
handoff between the base stations 40B and 44. In one embodiment, as
shown in the flow chart of FIG. 4, the network controller creates a
combined neighbor list which is transferred to the remote unit via
base stations 40B and 44 in block 120. For example, Table V shows
the combined neighbor list.
5TABLE V Reference B. S. ID # B. S. PN Offset Window Size PN Code
40A 40B 459 128 3 40C 40B 873 128 3 44 40B 0 Infinity 9 46 40B 0
Infinity 11 48 40B 0 Infinity 2 48 44 0 Infinity 2 46 44 4096 2048
11 42 44 5120 256 9 40B 44 0 Infinity 3
[0063] In block 122, the remote unit removes entries corresponding
to the active base station 40B and 44 as shown in Table VI. In
another embodiment, the entries corresponding to the active base
stations remain on the neighbor list but are simply excluded from
the searching process.
6TABLE VI Reference B. S. ID # B. S. PN Offset Window Size PN Code
40A 40B 459 128 3 40C 40B 873 128 3 46 40B 0 Infinity 11 48 40B 0
Infinity 2 48 44 0 Infinity 2 46 44 4096 2048 11 42 44 5120 256
9
[0064] In block 124, the remote unit aligns the timing reference to
one of the active base stations such as the base station 40B. Note
that the relative timing of signals received from two active base
stations as received at the remote unit is readily determined by
the remote unit. For example, in this case we assume that the
signal received from the base station 40B is measured to be offset
by 2200 PN chips from the signal received from the base station 44
and thus add 2200 chips to the PN sequence offsets specified with
respect to the base station 44. The resulting entries are shown in
Table VII.
7TABLE VII Reference B. S. ID # B. S. PN Offset Window Size PN Code
40A 40B 459 128 3 40C 40B 873 128 3 46 40B 0 Infinity 11 48 40B 0
Infinity 2 48 40B 2200(or 0) Infinity 2 46 40B 6296 2048 11 42 40B
7320 256 9
[0065] In block 126, any overlap in the values specified in the
table are removed. The results are shown in Table VIII. The overlap
in values can be determined by taking the intersection of the
search windows corresponding to a common base station entry.
8TABLE VIII Reference B. S. ID # B. S. PN Offset Window Size PN
Code 40A 40B 459 128 3 40C 40B 873 128 3 48 40B 0 Infinity 2 46 40B
6296 2048 11 42 40B 7320 256 9
[0066] In the example of Table VIII, the resultant window overlap
areas for the base stations 46 and 48 are equal to smallest window
size before combination. This is not always the case. For instance,
in an example not shown in Table VIII, for two entries
corresponding to a common neighbor base station with reference to a
common active base station, if the first entry specifies a 128 PN
chip search window centered about 3093 (or, equivalently, a search
window from 3029 to 3157) and a second entry specifies a 2048 PN
chip search window centered about 4096 (or, equivalently, a search
window from 3072 to 5120), we get a common range of 3072 to 3157.
In the case where an entry appears on the neighbor list more than
twice, a common, continuous search window can be determined by
taking the intersection of all the search window entries
corresponding to the common base station. In one embodiment, the
intersection of the search windows is increased or decreased by
some small amount. In another embodiment, the intersection is
rounded up or down to conform to a set of standard search window
ranges.
[0067] Although blocks 124 and 126 are shown above as distinct
operations for clarity purposes, in an actual embodiment, the
activities of blocks 124 and 126 can be carried out in a single
step without explicitly creating the information given in Table VI.
In addition, when creating the composite neighbor list, the remote
unit or network controller need not create an explicit list in any
particular form so long as the available information is used to
effectively limit the search space.
[0068] In the scenario just described with reference to FIG. 3, the
neighbor lists are combined in part in the remote unit. This same
type of combination can occur in the network controller, provided
that the relative timing offsets between the signal from the active
base station is reported from the remote unit to the network
controller. Even if the relative timing offsets are not reported to
the network controller, some level of combining can be done. For
example, if one entry specifies an infinite search window for a
particular base station and another entry specifies a smaller
window, the smaller window can always be used. Neighbor list
combinations in the network controller results in a reduction of
information transferred over the air which is advantageous to
system capacity. However, combining lists at the network controller
can also increase backhaul signaling which is required.
[0069] Each frame of data which is transferred over the forward
link to the remote unit is comprised of a series of symbols. When a
remote unit is in soft handoff between two base stations, it
combines the signals received from each base station on a symbol by
symbol basis. The combination of signals from multiple base
stations is described in detail in the referenced issued patents.
If the information from each base station is received at the remote
unit at a different time, the earlier arriving information must be
buffered until the latest arriving information is received. It is
advantageous to limit the amount of buffered data, both for ease of
implementation and to limit the cumulative delay.
[0070] If two base stations are asynchronous with respect to one
another, when the remote unit detects the pilot signal of a
neighboring base station, it can detect the relative time offset
between the two base stations only within the duration of the PN
sequence. The remote unit cannot discern the absolute timing
difference between the base stations. Stated another way, if the
relative timing offset between two asynchronous base stations is
greater than the period of the PN sequence, the remote unit detects
only the modulo portion of the timing offset between the two base
stations. For example, if the PN sequence repeats every 10 ms and
the time offset between the neighboring base station and the active
base station is 14 ms, the remote unit detects only a 4 ms offset
between the two base stations. If the remote unit enters soft
handoff between these two base stations and the information
transmitted from the newly active base station is offset from the
information transmitted by the originally active base station by
only 4 ms, on average, the remote unit buffers 10 ms worth of data
from the time advanced base station before the corresponding
information is received from the base station with the greatest
delay. The same type of result occurs if the timing between two
base stations is offset by 104 ms except that 100 ms of data must
be buffered at the remote unit. Such relatively large timing errors
are to be expected when two base station are asynchronous with
respect to one another. As noted above, such delays are undesirable
in the system and increase the buffering requirements of the remote
unit. According to the operation described below, such buffering is
avoided by aligning the transmissions from the base stations so
that the signals arrive at approximately the same time. In
addition, ambiguity in the frame timing must be resolved before the
remote unit can determine which symbols to combine with one
another.
[0071] Once frame alignment is established, the timing of the
forward link signals transmitted from the active base stations are
more finely adjusted so that the signals arrive at the remote unit
aligned to the symbol level. Due to the multipath and variable path
delays, the forward link signals seldom arrive at the remote unit
at exactly the same time. However, using time alignment techniques
as described in IS-95, timing on the order of a few symbols can be
achieved in most systems.
[0072] The frame synchronization of asynchronous base stations can
be determined before the remote unit enters soft handoff between
two asynchronous base stations. The frame synchronization of a base
station can be determined by the remote unit based upon an overhead
channel such as a broadcast channel transmitted from a base
station. FIG. 4 is a flow chart showing the process of entering
soft handoff between two asynchronous base stations. In FIG. 4, we
shall assume that the remote unit has established communications
with a base station A and that the base station A and a base
station B are asynchronous with respect to one another.
[0073] In block 100, the remote unit detects a rising pilot signal
level from the base station B. Such detection can result from
searching over a search space designated in a neighbor list as
specified above. The remote unit transmits a message identifying
the base station B as a candidate base station in block 102. In
block 104, the network controller receives the message via the base
station A. The network controller determines whether resources are
available in the base station B to service the remote unit. If such
resources are available, the network controller directs the remote
unit to add the base station B to a selected set by sending a
message via the base station A. The selected set comprises entries
corresponding to base stations which have been promoted from the
candidate set but about which frame synchronization information is
not available.
[0074] In block 106, the remote unit receives the message and adds
an entry corresponding to the base station B to the selected set.
The remote unit then monitors an overhead channel from the base
station B in order to determine the current frame alignment if such
alignment is unknown. The overhead channel is typically a broadcast
channel which carries base station and system information to remote
units within the coverage area of the base station.
[0075] In block 108, the remote unit transmits a message
identifying the relative frame synchronization of the base station
B. In block 110, the network controller receives the message and
directs the base station B to begin transmitting signals to the
remote unit according to the frame synchronization information such
that signals from the base station A and signals from the base
station B arrive at the remote unit frame synchronized. The network
controller also directs the base station B to search for the remote
unit transmitted signal. In block 112, the network controller
directs the base station A to transmit a message to the remote unit
designating the base station B as a member of the active set. In
block 114, the remote unit receives the message, acquires the base
station B transmitted signal and begins diversity combining the
signals transmitted by base stations A and B.
[0076] Notice that a new set of base stations has been created in
block 104: the selected set. The selected set designates base
stations which will be added to the active set upon the receipt of
the relative frame synchronization information. In order to monitor
the broadcast channel, precious remote unit resources must be
expended on the task. Therefore, in order to efficiently use the
remote unit resources, it is advantageous to avoid monitoring the
broadcast channel unless the network controller has determined that
the candidate base station has the available resources to become an
active base station. For this reason, in one embodiment, it is
advantageous if frame synchronization information is not collected
for every member of the neighbor set.
[0077] In another embodiment, the frame synchronization information
is collected corresponding to entries on the neighbor list without
the use of a selected set. For example, the remote unit can
determine the frame synchronization information just before or
shortly after a base station becomes a member of the candidate set.
Alternatively, the remote unit can determine frame synchronization
information of entries in the neighbor list having a signal
strength that exceeds some threshold which is less than the
threshold used to determine eligibility for the candidate set.
[0078] The selected set operation shown in FIG. 4 need not be
executed for base stations which are purely synchronous or frame
synchronous with respect to one or more of the active base
stations. The process shown in FIG. 4 may be implemented for a
third, fourth or greater number of base stations if the subsequent
base stations are asynchronous with respect to any of the active
base stations. If an additional purely synchronous or frame
synchronous base station is detected by the remote unit, it can
directly become a member of the active set from the candidate set
without the intermediate step of becoming a member of the selected
set.
[0079] In some systems, a frame-worth of symbols is transferred
from the network controller to each active base station at the same
time. Thus, a base station can determine when to transmit the
frames with an accuracy approximately equal to the variation in
relative propagation delay associated with the backhaul which
connects the base stations to the network controller. If such
relative delays are less than the frame duration, the base station
can derive coarse timing based upon the arrival time of the frames
from the network controller and more precise timing based upon
feedback received from the remote unit.
[0080] Even if the relative delays are larger than the duration of
a frame, the remote unit can attempt to combine symbols from
multiple base stations at a variety of frame offsets until it finds
the frame offset which generates the lowest error rate. Once the
relative synchronization is determined, in one embodiment, the
remote unit can report to the base stations and the base stations
can align future transmissions. This type of operation does not
alleviate the need for additional buffering in the remote unit but
it eliminates the need for the remote unit to monitor an additional
base station signal to determine the frame number as described
below. In addition, once frame synchronization is established, this
method eliminates excessive delay.
[0081] A block diagram of the remote unit 52 suitable for use in
conjunction with the just mentioned operation as well as with
operation according to other embodiments disclosed herein is shown
in FIG. 5. Referring to that FIG. it is seen that the remote unit
52 receives forward link signal via an antenna 158. A radio
frequency (RF) and analog receiver 160 receives the forward link
signal and converts the signal to digital samples according to well
known techniques. The digital samples are distributed to a series
of demodulation elements 162A-162N. Information concerning the
structure and operation of the demodulation elements 162A-162N and
demodulators in general can be found in U.S. Pat. No. 5,764,687,
entitled "MOBILE DEMODULATOR ARCHITECTURE FOR A SPREAD SPECTRUM
MULTIPLE ACCESS SYSTEM" and U.S. Pat. No. 5,109,390, entitled
"DIVERSITY RECEIVER IN A CDMA CELLULAR TELEPHONE SYSTEM," assigned
to the assignee hereof and incorporated herein by reference. Each
demodulation element 162A-162N can be assigned to a forward link
signal instance from any one of the active base stations. The
outputs of the demodulation elements 162A-162N are coupled,
respectively, to variable delay unit 164A-164N. The variable delay
units 164A-164N function to time align the output of each of the
demodulation elements 162A-162N so that they can be combined. The
variable delay units 164A-164N can be embodied as a data storage
unit. The amount of delay inserted in each path is determined by a
remote unit controller 168.
[0082] The outputs of the variable delay units 164A-164N are
coupled to a rate determination and combiner block 166. The rate
determination and combiner block 166 can operate according to well
known combination techniques such as those disclosed in U.S. Pat.
No. 5,566,206, entitled "METHOD AND APPARATUS FOR DETERMINING DATA
RATE OF TRANSMITTED VARIABLE DATA IN A COMMUNICATIONS RECEIVER,"
which is assigned to the assignee hereof and incorporated herein by
reference. The output of the rate determination and combiner 166 is
coupled to the remote unit controller 168 as well as to further
signal processing units (not shown) such as a vocoder or modem. In
one embodiment, the variable delay units 164A-164N are embodied as
a circular buffer that is accessed by the demodulation element
162A-162N as described in the above referenced patents.
[0083] The controller 168 is also coupled to the output of the rate
determination and combiner block 166. If the information carried on
the forward link transmission carries messages from the network
controller, the remote unit controller 168 extracts those messages
from the output of the rate determination and combiner 166 and
responds to them. For example, the controller 168 extracts the
information from the forward link transmission concerning the
active, selected and neighbor sets. In the embodiment given above,
if the forward link transmission comprises a message designating a
selected set, the controller 168 commands at least some of the
demodulation elements 162A-162N to demodulate an overhead channel
from the designated base station. The controller 168 extracts the
frame synchronization information from the output of the rate
determination and combiner 166 and determines frame synchronization
of the designated base station. In one embodiment, the remote unit
simply forwards a frame serial number received from the target base
station to the network controller and the network controller
determines the actual frame synchronization according to well known
techniques.
[0084] The controller 168 is also coupled to a memory unit 170
which can be used to store information. In addition, the controller
168 is coupled to a transmitter 172 which receives digital data for
transmission from further signal processing units (not shown) such
as a vocoder or modem. The transmitter 172 creates signals for
transmission over the reverse link. The controller 168 can also
create messages for transmission to the network controller via the
transmitter 172. For example, the remote unit controller 168 can
create a message informing the network controller of the relative
delay between signals received from two active base stations or
concerning the frame synchronization information of a base station
in the selected set.
[0085] The output of the RF and analog receiver 160 is also coupled
to a pilot strength measurement circuit 156. In one embodiment, the
pilot strength measurement circuit 156 is comprised of a set of
searching elements or a searcher front end as described in the
above referenced patents such as U.S. Pat. No. 5,764,687. The
output of the pilot strength measurement circuit 156 is coupled to
the remote unit controller 168. In addition, the remote unit
controller 168 controls the operation of the pilot strength
measurement circuit 156. For example the remote unit controller 168
selects the PN sequence, PN sequence offset and window size over
which the pilot strength measurement circuit 156 searches for the
pilot signals from the base stations. As discussed above, in one
embodiment, the controller 168 allocates the searching resources
within the pilot strength measurement circuit 156 in accordance
with the information in the neighbor list.
[0086] FIG. 6 is a flow chart detailing the alternative frame
alignment process referred to above where no selected set is
created. According to FIG. 6, an entry corresponding to a candidate
base station is moved directly to the active set even if the frame
alignment of the base station with respect to the other active base
stations is unknown. The frame alignment process illustrated in
FIG. 6 is remote unit assisted in that the remote unit determines
the frame alignment without reference to information received
explicitly from the base station or network controller. Thus, the
embodiment of FIG. 6 avoids the round trip delay associated
transmission of information between the remote unit and the base
station which is incurred with the use of the selected set.
[0087] In block 140, the remote unit receives and demodulates a
forward link signal from a first base station having a time
alignment of X. If the first base station is the only active base
station, the time alignment of the first base station can be
assumed to be the timing reference for the remote unit in which
case X is conveniently set to 0.
[0088] In block 142, the remote unit receives and demodulates a
forward link signal from a second base station having a time
alignment of Y. If the second base station is the newly added
active base station, Y is typically some positive or negative
non-zero value which is the modulo result of comparing the PN
sequence timing of the first base station to the PN sequence timing
of the second base station. Thus, the time alignment Y specifies
the timing of the frame boundaries but does not specify the
absolute frame timing of the second base station. Thus, in a
typical embodiment, if the value of X is 0, the absolute value of Y
is less than the PN sequence period.
[0089] The demodulated data corresponding to one signal instance is
stored (such as by one of the variable delay units 164) until the
timing of the signals is aligned with respect to the frame
boundaries using a frame offset Z in block 146. However, note that
the absolute frame alignment is not yet known so the frame offset Z
may align the signals on frame boundaries offset from one another
by an integer number of frames. For example, in one embodiment, the
value of Z is initially zero and the relative offset of the signals
is assumed to be the difference between X and Y.
[0090] In block 148, the signals are combined such as by the rate
determination and combiner block 166. Note that when the frame
boundaries are offset from one another, the information carried by
the symbols is generally different, thus degrading receiver
performance by the addition of arbitrary energy. When the frame
boundaries are aligned, the information carried by the symbols is
the same, thus increasing the energy available to the receiver and
increasing the performance of the receiver in comparison with
demodulating only the signal from the first base station. According
to well known techniques such as those taught in the above
referenced U.S. Pat. No. 5,566,206, the rate determination and
combiner block 166 determines an error rate or other performance
indicator or indicators with respect to received signals. In block
150, if the performance indicator is within acceptable limits, it
is assumed that the frame alignment is proper and the process
terminates. For example, acceptable limits can be determined by
comparison of error rates before and after combination is begun. If
the performance indicator is not within acceptable limits, flow
continues to block 152.
[0091] In one embodiment, several frame offset hypotheses are
tested and the results are compared in a similar manner as
described in the above reference U.S. Pat. No. 5,566,206. In one
embodiment described in the patent which can be directly applied to
the invention, two or more performance indications are compared
from multiple rate hypothesis to determine the most likely
transmitted data rate. According to the invention, data can be
collected for a series of frame alignment hypothesis and the
results can be compared to determine the most likely correct frame
alignment.
[0092] In block 152, the delay inserted by the variable delay units
164 is offset from a previous value by some integer multiple of a
PN sequence period. The process repeats using different frame
offsets until an acceptable performance indication is produced. For
example, in one embodiment the value of n is incremented in a
linear fashion alternating between positive and negative values
such as n=0, 1, -1, 2, -2, 3 etc. In addition, this process,
beginning again with block 140, can be repeated for additional base
stations as more or different base stations are added to the active
set in order to align the timing of subsequent active base
stations.
[0093] Although the principles are described above with reference
to a system in which several classes of synchronization are used,
the principles can be applied generally to systems in which all of
the base stations have the same class of synchronization. In
addition, although the individual PN sequences are referred to as
"unique," the sequences can be re-used in base stations which are
operating at a different frequency or which are separated by a
sufficient path loss (general evidenced by a sufficient
geographical distance) so that the base station signals could not
be simultaneously detectable by any remote unit.
[0094] The techniques described above can also be applied to
control of hard "break-before-make" handoff. Although base stations
in an IS-95 continually transmit a pilot signal, the teachings of
the invention are directly applicable to systems in which the pilot
signal is transmitted intermittently. The embodiments shown above
with reference to neighboring base station can be directly applied
to those base station which are considered to be remaining set base
stations. For example, in one embodiment, the remaining set base
stations are included within the neighbor set but are allocated a
smaller portion of the searching capabilities of the remote
unit.
[0095] A remote unit operating according to the invention can
include embodiments which are implemented in application specific
integrated circuits (ASIC), with discrete logical components, in
software which is executed on a microprocessor or in firmware.
Likewise, a base station or network controller operating according
to the invention can include embodiments which are implemented in
application specific integrated circuits (ASIC), with discrete
logical components, in software which is executed on a
microprocessor or in firmware.
[0096] The invention may be embodied in other specific forms
without departing from its spirit or essential characteristics. The
described embodiment is to be considered in all respects only as
illustrative and not restrictive and the scope of the invention is,
therefore, indicated by the appended claims rather than the
foregoing description. All changes which come within the meaning
and range of equivalency of the claims are to be embraced within
their scope.
* * * * *